This invention relates generally to an improved globular or spherical kinematic joint which, as known to those skilled in the art, provides, kinematically, three degrees of freedom of rotation; spherical kinematic joints are commonly used in machinery applications such as heavy machinery and business machinery and are also commonly used in various automotive applications such as tie rod connections. Such joints typically include a member providing a ball or spherical head, an intermediate bearing insert providing a cavity or seat for receiving the spherical head and a third member providing another cavity or seat for receiving the bearing insert with the ball or spherical head residing therein. Many applications require that such joint be capable of resisting dislocation when it is at its extreme limits of rotational movement and that it resist separation under a load tending to pull the connected joint members apart. The structure of such joints is well known to those skilled in the art and it is also well known that such joints may be comprised of many different structural elements.
As is further known, it is often desired that such joints eliminate or minimize axial play or slop between the connected joint members. Joints illustrating this may be seen in U.S. Pat. Nos. 1,891,804 and 1,894,309 to W. A. Flumerfelt issued Dec. 20, 1932 and Jan. 17, 1933, respectively; U.S. Pat. No. 1,985,728 to G. B. Ingersoll issued Dec. 25, 1934; U.S. Pat. No. 3,220,755 to Gottschald et al. issued Nov. 30, 1965; and U.S. Pat. No. 3,401,965, to W. C. Wehner issued Sept. 17, 1968. All of the joints disclosed in these patents have provision for minimizing or eliminating axial play. These patents further illustrate two different types of spherical kinematic joint structure or construction. In the Flumerfelt and Ingersoll patents, the bearing inserts comprise separate bearing insert segments which are assembled over the ball or spherical head and are then inserted into a housing where they are retained by various means. The Wehner patent shows a different type of joint structure or construction which includes a load carrying bearing insert and a retaining collar for preventing dislocation and tensile separation of the connected joint members. The Gottschald et al. patent illustrates a unitary construction wherein the bearing insert contains multiple splits of the front face near the opening providing a multiplicity of fingers or arcuate sections which expand outwardly upon being forced into engagement with the ball or spherical head; after such assembly, the bearing insert and ball are then inserted into the cavity or seat of the third member whereby the third member prevents outward expansion of the fingers thereby trapping the bearing insert on the ball. Additional or secondary means are then provided to hold the bearing insert in the cavity of the third member thereby providing a joint that resists tensile separation and dislocation.
Recently, spherical kinematic joints have found application in human joint prostheses. In prosthetic applications, ease of assembly and disassembly of the joint elements or members are of great importance. The use of an intermediate bearing insert comprised of a plurality of bearing segments unconnected when not assembled in the joint is undesirable in prosthetic applications since handling and holding of the bearing segments against the head while assembling the head and bearing segments with the third member can be awkward, particularly to an operating surgeon working inside of a surgical cavity. Although a bearing insert having flexible fingers or arcuate sections reduces the amount of assembly force that would be required were the fingers not present, such flexible fingers can still be quite awkward to the operating surgeon since to provide the required or desired dislocation and separation resistance and strength the fingers must be quite stiff which, in turn, requires that considerable assembly force still be applied by the operating surgeon.
As is well known to those skilled in the art, in prosthetic joint applications, low assembly force and ease of assembly are of great importance because of the need to minimize assembly time and because of the difficulties under which the surgeon must operate. Often, the field of view is obscured and joint elements or members are slippery because of coating with blood and fat which can result in dropping of the joint elements thereby causing a further problem since such joints must be sterile and upon dropping they must be resterilized, or replaced, while the surgical cavity remains open. Further, on occasion, because of surgical emergency or other intra-operative difficulties, it is desirable to be able to disassemble the joint members, perferably in a manner which avoids the same difficulties of assembly noted above, i.e., excessive disassembly forces and multiple unconnected bearing insert members which are difficult to handle. Thus, joints which are satisfactory for machinery applications where fixtures and other tooling may be conveniently used to assemble joint members need improvement for their effective utilization in prosthetic joints.
Patents disclosing prosthetic joints include U.S. Pat. No. 3,813,699 to Richard P. Giliberty, issued June 4, 1974; U.S. Pat. No. 3,863,273 to Robert G. Averill, issued Feb. 4, 1975; U.S. Pat. No. 4,044,403 to D'Erricho issued August, 1977; and U.S. Pat. No. 4,241,463 to Alex Khovaylo issued Dec. 30, 1980. The prosthetic joint disclosed in the Giliberty patent provides a prosthetic joint which is resistant to dislocation and separation and partially avoids the assembly problems noted above by providing a unitary structure including both the bearing insert and the acetabular cup thereby minimizing the handling of various joint members during the assembly and disassembly process. However, the unitary structure of the bearing insert and the acetabular cup does not allow for ease of assembly and disassembly with high separation resistance, since the opening into the spherical cavity of the bearing insert which receives the ball-shaped femoral head is incapable of substantial outward expansion or flexing upon being forced into engagement with the ball-shaped head and therefore the Giliberty prosthetic joint requires either extremely high assembly or disassembly forces and/or relatively low resistance to dislocation and separation forces. The prosthetic joint disclosed in the Averill patent partially overcomes the problem of excessively high assembly or disassembly forces by utilizing, as in the Gottschald et al. patent, an intermediate bearing insert provided with a plurality of arcuate sections at the entry into the spherical seat for receiving the ball which arcuate sections upon being forced into engagement with the femoral head still require a generally undesirably high manual force to expand outwardly to permit entry of the ball-shaped head into the spherical seat of the intermediate bearing insert. The Averill prosthetic joint also utilizes a tab provided on the bearing insert to prevent separation of the bearing insert from the acetabular cup; however, such structure introduces undesirable axial play into the prosthetic joint. Similarly, the prosthetic joint disclosed in the D'Erricho patent is also a partial solution to the above-noted assembly problem in that the assembly forces required to assemble the joint members are reduced below those required to assemble the joint members of the Giliberty prosthesis given a predetermined dislocation and separation resistance; this is accomplished by the use of flexible lips provided on the bearing insert acting against the spherical head or ball of the first member and/or the cavity in the third member or acetabular cup. Such joint construction, however, is not readily disassemblable without some damage to the bearing insert and does not provide the desirable combination of low force of manual assembly and high separation resistance needed by an operating surgeon intra-operatively. The Khovaylo prosthetic joint provides structure which allows for low assembly force of the bearing insert onto the head but employs a solution similar to that disclosed in the D'Erricho patent for assembly of the third member or acetabular cup onto the bearing insert. Further, the internal retaining ring of the Khovaylo prosthetic joint operates in a manner which produces end play between the spherical ball and the bearing insert. Further, the Khovaylo prosthetic joint does not provide for convenient assembly or disassembly of the bearing insert from the third member or acetabular cup intra-operatively. As is further known to those skilled in the prosthetic joint art, physically separate acetabular cups and bearing inserts are desirable in prosthetic joint applications in order to minimize the cost of stocking various sized prostheses and in order to minimize sterilization costs associated with such joints.
As is still further known to those skilled in the art, the bearing inserts of the above-noted patents, provide an overlap which, upon the ball-shaped head being received within the bearing insert and the bearing insert with the ball-shaped head seated therein being received within the insert housing, prevents dislocation of the ball-shaped head from the bearing insert during rotational movement between the ball-shaped head and the insert housing cup.